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From Ted Dunning <ted.dunn...@gmail.com>
Subject Re: Leader election and leader operation based on zookeeper
Date Sat, 21 Sep 2019 16:56:54 GMT
WHat I suggested is almost exactly what Jordan suggested. I should have
read the rest of the thread before posting.

On Sat, Sep 21, 2019 at 9:54 AM Ted Dunning <ted.dunning@gmail.com> wrote:

> I would suggest that using an epoch number stored in ZK might be helpful.
> Every operation that the master takes could be made conditional on the
> epoch number using a multi-transaction.
> Unfortunately, as you say, you have to have the update of the epoch be
> atomic with becoming leader.
> The natural way to do this is to have an update of an epoch file be part
> of the leader election, but that probably isn't possible using Curator. The
> way I would tend to do it would be have a persistent file that is updated
> atomically as part of leader election. The version of that persistent file
> could then be used as the epoch number. All updates to files that are gated
> on the epoch number would only proceed if no other master has been elected,
> at least if you use the sync option.
> On Fri, Sep 20, 2019 at 1:31 AM Zili Chen <wander4096@gmail.com> wrote:
>> Hi ZooKeepers,
>> Recently there is an ongoing refactor[1] in Flink community aimed at
>> overcoming several inconsistent state issues on ZK we have met. I come
>> here to share our design of leader election and leader operation. For
>> leader operation, it is operation that should be committed only if the
>> contender is the leader. Also CC Curator mailing list because it also
>> contains the reason why we cannot JUST use Curator.
>> The rule we want to keep is
>> **Writes on ZK must be committed only if the contender is the leader**
>> We represent contender by an individual ZK client. At the moment we use
>> Curator for leader election so the algorithm is the same as the
>> optimized version in this page[2].
>> The problem is that this algorithm only take care of leader election but
>> is indifferent to subsequent operations. Consider the scenario below:
>> 1. contender-1 becomes the leader
>> 2. contender-1 proposes a create txn-1
>> 3. sender thread suspended for full gc
>> 4. contender-1 lost leadership and contender-2 becomes the leader
>> 5. contender-1 recovers from full gc, before it reacts to revoke
>> leadership event, txn-1 retried and sent to ZK.
>> Without other guard txn will success on ZK and thus contender-1 commit
>> a write operation even if it is no longer the leader. This issue is
>> also documented in this note[3].
>> To overcome this issue instead of just saying that we're unfortunate,
>> we draft two possible solution.
>> The first is document here[4]. Briefly, when the contender becomes the
>> leader, we memorize the latch path at that moment. And for
>> subsequent operations, we do in a transaction first checking the
>> existence of the latch path. Leadership is only switched if the latch
>> gone, and all operations will fail if the latch gone.
>> The second is still rough. Basically it relies on session expire
>> mechanism in ZK. We will adopt the unoptimized version in the
>> recipe[2] given that in our scenario there are only few contenders
>> at the same time. Thus we create /leader node as ephemeral znode with
>> leader information and when session expired we think leadership is
>> revoked and terminate the contender. Asynchronous write operations
>> should not succeed because they will all fail on session expire.
>> We cannot adopt 1 using Curator because it doesn't expose the latch
>> path(which is added recently, but not in the version we use); we
>> cannot adopt 2 using Curator because although we have to retry on
>> connection loss but we don't want to retry on session expire. Curator
>> always creates a new client on session expire and retry the operation.
>> I'd like to learn from ZooKeeper community that 1. is there any
>> potential risk if we eventually adopt option 1 or option 2? 2. is
>> there any other solution we can adopt?
>> Best,
>> tison.
>> [1] https://issues.apache.org/jira/browse/FLINK-10333
>> [2]
>> https://zookeeper.apache.org/doc/current/recipes.html#sc_leaderElection
>> [3] https://cwiki.apache.org/confluence/display/CURATOR/TN10
>> [4]
>> https://docs.google.com/document/d/1cBY1t0k5g1xNqzyfZby3LcPu4t-wpx57G1xf-nmWrCo/edit?usp=sharing

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